Method of Making Anterior Dental Restorations from Sintered Preforms

ABSTRACT

A method is provided for shaping a custom anterior dental restoration from a preform, wherein the preform comprises a preform body and a preform stem. A method is further disclosed for generating one or more nesting positions for the restoration design within the geometry of the preform body relative to the position of the preform stem. A method is further disclosed for generating machining instructions based on the selected nesting position to optimize machining for chair-side applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application Ser. No. 62/929,303, filed on Nov. 1, 2019, theentirety of which application is hereby incorporated by reference in itsentirety.

BACKGROUND

Ceramic materials known for use in the field of dentistry provide highstrength restorations such as crowns, bridges, and the like. Someceramic materials have flexural strength values exceeding 800 MPa whenfully sintered, resulting in restorations that are resistant tochipping, breakage and wear. Material advances provide enhancedaesthetics in color and translucency while maintaining acceptablestrength, and restorations may be manufactured from these materials in acost effective manner.

Dental restorations created by computer assisted design processes may bemilled by CAM processes from porous ceramic materials in the green orbisque ceramic stage, using an enlargement factor to accommodatereduction in overall size upon heating to full density. After milling,the porous restoration design is sintered to full density to produce afinal restoration. Disadvantageously, the separate steps of milling theporous ceramic dental design and sintering the milled shape to form thefinal dental restoration, may preclude dentists from making chair-sideceramic restorations, increasing the amount of time a patient must waitfor repair.

To reduce the amount of material waste to make a restoration,US2006/0204932 discloses an assemblage or library of “smart” mill blankspre-configured into geometries and sizes that closely resemble the finaldental parts. Material waste may be reduced compared to traditional millblanks that have a single size and shape, which is desirable when usingprecious or semi-precious materials. The smart mill blank library isdescribed as comprising a series of blanks with geometries that differother than by scale, and preferably having at most, one symmetric plane.The blank is mounted in a shaping apparatus by a milling holder that hasan orientation-specific attachment key for the milling machine.

Methods of making ceramic restorations from near net shape millableblanks are also known, for example, from commonly owned U.S. Pat. No.9,597,265, which is hereby incorporated by reference in its entirety. Inthis document, a kit is disclosed containing millable blanks of variousshapes, each shape designed to closely replicate a restoration shapethus minimizing material removal in chair-side processes. The kitcomprises a variety of shapes and shades of restoration blanks, as wellas chair-side software, and a chair-side milling machine to convertmillable blanks into finished, contoured restorations by a dentist.Methods for making dental restorations from sintered preforms aredescribed in commonly owned U.S. Pat. No. 10,258,440, issued Apr. 16,2019, which is hereby incorporated by reference in its entirety.

SUMMARY OF THE INVENTION

A method for making a custom dental restoration, such as a crown, from amachinable preform is disclosed. The methods are suitable for shapingmaterials that have sufficient strength and hardness properties intodental restorations that may be directly inserted into the mouth of apatient without the need for a further processing step to strengthen thematerial after it has been shaped. Methods and apparatus describedherein reduce the time required to prepare a finished dentalrestoration. Advantageously, fully sintered materials known for strengthand durability, such as sintered yttria-stabilized zirconia, may beshaped directly into restorations in chair-side applications or in alaboratory without requiring post-shaping sintering processes. A novelsintered, shaped preform and shaping tool are described, as well asunique nesting methods, machining strategies, and tool paths.

A sintered preform from which a final, custom anterior dentalrestoration is shaped comprises a body of sintered material and a stemprojecting from the center of the preform body. When used in a dentistoffice chair-side milling machine, the time to create a custom finishedproduct is significantly reduced. Unique features of the sinteredpreform specifically tailored for anterior dental restorationapplications include the size and shape of the preform body whichaccommodate most custom anterior dental restoration designs having areduce the amount of sintered material to be removed during the processof shaping a dental restoration. Advantageously, the shape of thepreform accommodates multiple options for nesting the dentalrestoration, and methods described herein for selecting nestingpositions based on stem placement options enable the generation ofunique tool paths for shaping dental restorations from sinteredmaterials. Material preforms are provided herein comprising stems thatprovide novel attachments to a mandrel for inserting in a millingmachine.

A method for making a custom anterior dental restoration comprisesdesigning a custom anterior dental restoration by a known CAD(computer-aided design) process, nesting a CAD dental restoration designwithin a computer model of a preform body, generating tool paths from amachining strategy and the positional information of the nestedrestoration design, and machining the sintered preform into the finalrestoration. In one embodiment, a method comprises nesting therestoration design within the preform body, wherein the preform stem ispositioned adjacent mesial or distal contact areas. A method furthercomprises generating at least two tool paths for shaping the incisalside of the anterior dental restoration or cavity side of an anteriordental restoration. In another embodiment, at least two tool paths areprovided for shaping the anterior dental restoration from the incisalside and at least two additional tool paths are provided for shaping thecavity side of the restoration, wherein a single tool path is providedfor a portion of the vestibular surface and a portion of the cavitysurface. In a further embodiment, a machining strategy is provided forreducing the diameter of the stem adjacent the dental restorationsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C. A graphic representation of an anterior dentalrestoration.

FIG. 2. A graphic representation of a dental restoration attached to astem of a preform.

FIGS. 3A and 3B. A graphic representation of a preform on a mandrelaccording to one embodiment.

FIG. 4. A graphic representation of a preform on a mandrel according toone embodiment.

FIG. 5. A graphic representation of a preform body and stem according toone embodiment.

FIG. 6. A graphic representation of a mandrel according to oneembodiment.

FIGS. 7A, 7B and 7C. Graphical representations of a preform according toone embodiment.

FIG. 8. A cross-sectional representation of a preform stem in a mandrelaccording to one embodiment.

FIGS. 9A, 9B and 9C. Graphical representations of nesting an anteriorrestoration design within a model of preform.

FIGS. 9D through 9H. Graphical representations of exemplary nestingpositions of an anterior restoration design relative to a stem of amodel of millable preform.

FIG. 10. A graphical representation of a lacing tool path for avestibular surface of an anterior dental restoration design.

FIGS. 11A, 11B and 11C. Graphical representations of a portion of a toolpath across a slope according to one embodiment.

FIGS. 12A through 12G. Graphical representations of machining strategiesfor generating tool paths for an anterior dental restoration.

FIGS. 13A through 13D. Graphical representations of machining strategiesfor milling an anterior dental restoration from a preform.

FIGS. 14A and 14B. A graphical representation of machining strategiesfor milling labial and lingual surfaces of an anterior dentalrestoration.

FIGS. 15A through 15B. Graphical representations of an exemplarymachining strategy for reducing a stem of a preform.

FIGS. 16A through 16D. Graphical representations of an exemplarymachining strategy for reducing a stem of a preform.

FIG. 17. A side view of a graphic representation of a grinding toolaccording to one embodiment.

DETAILED DESCRIPTION

A machinable, fully sintered ceramic preform is provided hereincomprising novel stem and mandrel attachments. Further, a method formaking a custom anterior dental restoration from a sintered ceramicpreform is described herein.

An anterior dental restoration milled from a machinable preform isillustrated in FIGS. 1A through 1C (100). Anterior dental restorationsmade from machinable blocks by methods provided herein replace orrestore mandibular and maxillary central incisors, lateral incisors andcuspids. Anterior teeth are referred to in the Universal NumberingSystem (also known as the “American system”) in field of dentistry bythe dental notation for permanent dentition as numbers 6 through 11 onthe maxilla and numbers 22 through 27 on the mandible of a patientwherein the midline is between 8 and 9 (upper) and 24 and 25 (lower). Insome embodiments, the machinable fully sintered ceramic preform may alsorestore the natural dentition of first bicuspids and/or secondarybicuspids, referred to by the dental notation for permanent teeth asnumbers 4, 5, 12, 13, 20, 21, 28 and 29.

An anterior dental restoration (100), such as crown, may have an incisalsurface (101) that comes into contact with opposing teeth duringocclusion. A fitting surface or cavity (102) fits on an abutment or apreparation tooth, a patient's tooth which outer surface has beenprepared, e.g., by removing a portion of the natural tooth. A gingivalmargin (103) comprises a region or edge between the fitting surface anda vestibular surface. The vestibular surface (104) comprises the outersurface of the restoration including a labial surface (105) that facesthe lips of a patient, and a lingual surface (106) facing the tongue.Vestibular side surfaces (107), include a mesial surface that is theside of the crown closest the midline of the patient's mandible ormaxilla and a distal surface that is the side of the tooth farthest fromthe midline, both of which may be proximal contact surfaces to adjacentteeth when installed in the mouth of a patient.

As illustrated in FIG. 2, in one embodiment, a fully sintered preformmay be machined chair-side in a dentist's office into a final dentalrestoration (200), such as a crown. The fully sintered preform may besecured in a chair-side milling machine by an attachment means (203).The sintered dental restoration (200) may be easily separated from apreform stem (201) at a first stem end (202) that has been reducedduring the milling process. After separating from the stem (201), thedental restoration (200) is secured directly into the mouth of a patientwithout requiring a post-process sintering step. A method is providedfor machining the sintered preform into a custom final dentalrestoration that reduces the time required to prepare a fully sinteredfinal dental restoration. The methods and apparatus disclosed hereincomprise novel features including a unique preform design, nestingstrategies, tool paths and machining strategies. In a furtherembodiment, a kit is provided that comprises a preform designed foranterior restorations, a grinding tool, computer programs or modules fornesting a restoration design within the preform, and machiningstrategies for milling a final restoration chair-side, without the needfor sintering after shaping the restoration.

Illustrated in FIGS. 3A and 3B, one embodiment of a preform (300)comprises a machinable, fully sintered ceramic body (301) from which adental restoration (200) is machined, and a stem (302) that projectsfrom the body (301). A sintered preform (300) is shown that has acircular-cylindrical sintered ceramic body (301) having a curved outersurface along a cylinder length (line A-A′, A-axis). In an exemplaryembodiment, the preform body is positioned in chairside mill so that theaxis of the cylinder length is in the same direction as the Z-axis of ashaping tool of a CNC machine. A center portion (305) of the sinteredceramic body (301) extends between bottom end (306) and a top end (307).In FIGS. 3A and 3B, the length axis (A-axis, line A-A′) of thecylindrical body (301) is substantially orthogonal to the length axis(C-axis, line C-C′) of the stem (302) in the y-axis direction. In thisembodiment, the stem projects from a stem contact point on asubstantially smooth, curved, outer surface (308) of the center portion(305) of the cylindrical body, and the stem (302) connects to the upperportion (303) of a mandrel (304) for attachment to a milling machine.

The outer diameter of a circular cross-section of the center portion(305) of a preform body from which the restoration design is shaped maybe from 9 mm to 20 mm, or from 9 mm to 16 mm, or 9 mm to 16 mm, or from9 mm to 12 mm, or at least 9 mm and less than 12 mm, such as from 9 mmto 11.5 mm, or 9 mm to 11 mm. The length of the preform body between thetop end and the bottom end is sufficient to accommodate the height ofmost dental restoration designs when measured, for example, from thehighest point of the incisal surface to the lowest point on a gingivalmargin; thus, the length of the preform body or the center portion ofthe preform body may be less than 20 mm, or less than 18 mm, or lessthan 16 mm, or less than 15 mm, or between about 10 mm and 20 mm. Insome embodiments, the ratio of the cross-sectional diameter of thecenter portion to the length of the preform body is greater than1.0:1.0, or 1:1, or less than 1:1, such as between 1:1 and 1:2.

The preform body may comprise a shape other than a cylinder, forexample, an ellipsoid cylinder, a polyhedron, curved polyhedron, acylinder with flattened surfaces, a square, a square with rounded edges,and the like. In one embodiment, the restoration design fits within thedimensions of the preform body for a complete rotation around therestoration design length axis. A preform body may have across-sectional geometry (parallel with top and bottom surfaces) with aninscribed circle diameter greater than 9 mm and a circumscribed circlediameter less than 12 mm at the stem contact point.

In some embodiments, the preform body is solid and the top end surfaceand/or bottom end surface are flat (FIG. 9A). As illustrated in FIGS. 3Aand 3B, optionally the top and bottom end surfaces also comprise ashaped edge (309). For example, a preform may comprise a chamfer orfillet between the curved outer surface (308) of the preform body (301)and top end surface (307) and/or bottom end surface (306). Asillustrated in FIG. 5, a filleted edge (505) may surround the bottom endsurface (506). Advantageously, forming a dental restoration from apreform having a shaped edge, may require less material removal,shortening the shaping time.

As exemplified in FIG. 5, optionally, the preform body (501) comprises apreform body cavity (503) that begins at a top surface, bottom endsurface (506), or both top and bottom, and extends into the preform bodywherein a preform body cavity surface is accessible by a shaping tool.The shape of each preform body cavity may comprise, but is not limitedto, an inverted cone, dome, cylinder, trough, or the like, or may havean irregular shape. An opening or breakout geometry of the preform bodycavity may have a width (or diameter, for example where the breakoutarea is circular) that is between about 20% and 80% of the outerdiameter or width of the center portion of the preform body. About 50%to about 80% of the surface area of a top end surface, a bottom endsurface, or a center portion cross-section, comprises the break-outdimension. The approximate preform cavity depth extending from the endsurface into the preform body may be between 5% and 50% of the length ofthe preform body.

As exemplified in FIGS. 3A and 3B, the preform stem (302) length (alongC-axis) is orthogonal to the length of the cylindrical body (alongA-axis). In some embodiments, the stem length is within about 30 degreesor within about 45 degrees of orthogonal, relative to the body. In oneexample, a first stem end (308) extends from the center portion (305) ofa cylindrical body (301), and the preform stem contact point isapproximately equidistant between the bottom end (306) and the top end(307) of the preform body. In some embodiments, prior to shaping thesintered preform, the length of a preform stem is greater than the widthof the stem at the first stem end (308) proximate the cylindrical body.For purposes herein, width may be used interchangeably with diameter,for example, in embodiments in which the stem has a circularcross-section. The preform stem length may be between about 3 mm andabout 12 mm, or between about 3 mm and about 10 mm prior to milling therestoration. In some embodiments, the preform stem length may be greaterthan about 3 mm, or greater than about 4 mm, greater than about 5 mm,greater than about 6 mm, or greater than about 8 mm. In one embodiment,the width (diameter) of the first stem end (308) is less than the width(diameter) of the second stem end (310) proximate the top portion (303)of the mandrel (304). The width (diameter) of the first stem end may beabout 1 mm to about to about 4 mm, or from 1 mm to about 3 mm, or about1.5 mm to about 3 mm, or 1.5 mm to about 2.5 mm, or less than about 4mm, or less than about 3 mm, or less than about 2.5 mm. In someembodiments, the ratio of stem length to the first stem end (308) widthor diameter (proximate the preform body) is greater than about 1.5:1, orgreater than about 2:1, or greater than about 3:1.

In one embodiment, the preform stem length is greater than the diameterof the shaping tool. The distance between the preform body (301) and thetop portion of the mandrel (303) provides a space equivalent to thelength of the stem for entry of a shaping tool near the first stem end(308), without the tool tip contacting the sintered preform material,and thereby reducing tool wear. The flex strength of the preform stem(302) at the first stem end (308) is sufficiently high to support thesintered preform (300) during machining from a sintered state, andsufficiently low for the finished restoration to easily be broken offfrom the stem, for example, by hand. The preform stem shape may be acylinder, tapered cylinder, cone, prism or the like, and the stem isconnected to the center portion of the preform body at the stem contactpoint by a first stem end.

Milling machines, such as chair-side mills suitable for milling thesintered preform into a final dental component have at least 3 axes,such as a 3+1 axis CNC machine. A suitable chair-side milling machineincludes, but is not limited to the TS150™ chair-side milling system(IOS Technologies, San Diego, Calif.), or a milling machine as describedin commonly owned U.S. Pat. No. 10,133,244, which is incorporated byreference herein in its entirety. The sintered preform (400) may besecured within a milling machine by fastening the preform to a mandrel.In one embodiment illustrated in FIG. 4, a preform (400) comprising amillable body (401) and preform stem (402) may further comprise anattaching member (403) for attaching the preform to a mandrel (404). Theattaching member (403) may be shaped as a rectangle, circle, or square,having a substantially flat surface for adhesive attachment to amandrel. The mandrel (404) may be secured within a milling machine witha locking mechanism (405), such as locking pin and hole elements.

In a further embodiment illustrated in FIGS. 6, 7A, 7B, 7C and 8, thepreform stem is inserted into a mandrel (600) through a hole (601) onthe top portion (602) of the mandrel, and secured, for example, by aninterference fit, such as a press or friction fit, adhesive ormechanical locking mechanism. In one embodiment of a preform (700),exemplified in FIGS. 7A, 7B and 7C, the preform body (701) is attachedto a preform stem (702) having a second stem end (703) that comprises atleast one groove (704), at least one ridge (706), or both, on the outersurface of the second stem end (703), for attachment to a mating end ofthe mandrel. The groove (704) and/or ridge (706) may engage with acomplementary convex or convex surface feature on the inner surface ofthe mandrel to form a locking mechanism. In an alternative embodiment,the at least one ridge (706) is comprised of sintered zirconia cuts ordepresses the inner surface of a mandrel made from a softer material,such as a polymer (e.g., nylon) forming an anti-rotational press fitconnection.

In a further embodiment, attachment between the preform stem and themandrel may be strengthened by the addition of an adhesive. Optionally,surface features, such as one or more channels (707) or grooves (704),are milled into the stem to hold adhesive and increase the bondingsurface area of the stem outer surface area. FIG. 8 illustrates across-sectional view of an interference fit between the preform stem(801) engaged within a mandrel (802) wherein a portion of the mandrelinner surface (804) is deformed by a ridge (706) forming ananti-rotational fit, and an adhesive connection (800) comprising anadhesive within stem channels (707) and within a gap (803) between thestem (801) and mandrel (802). Suitable adhesives include, and are notlimited to, light and heat curable adhesive resin, such as a two-partepoxy composition (e.g., UHU® Plus 300, two-part epoxy resin, heavy dutyapplications, UHU GmbH & Co., Búhl, Germany).

The anterior dental restoration design may be created manually by anoperator, or automatically proposed, in a dental restoration CAD system.Known design systems, such as IOS FASTDESIGN™ System (IOS Technologies,San Diego, Calif.), are suitable for designing dental restorations foruse herein, as well as methods disclosed in commonly owned U.S. Pat.Pub. 2015/0056576, U.S. Pat. No. 10,248,885, and U.S. Pat. No.10,157,330, the disclosure of each is hereby incorporated herein byreference in the entirety. Anatomical information about the patient'stooth preparation, and optionally, surrounding teeth and the patient'soriginal tooth structure, may be obtained from an intraoral scan orscans of physical impressions taken from the patient. Data are collectedand stored in a computer or computer storage media to form a digitalrestoration design. In further embodiments, methods are provided forgenerating machining instructions to mill the anterior dentalrestoration from the millable preform body in CAD/CAM-based systems.

A computer-implemented method for nesting a computer model of thepatient-specific restoration within a computer model of the preformgeometry is provided. The computer model of a patient's dentalrestoration design may be selectively nested in one or more optionalpositions within a computer model of the preform to establish optimalmachining conditions for a selected CNC machine, milling or grindingtool and/or machine coolant system, and preform materials. Positionaldata of the nested restoration design may be provided to the CAM systemto calculate tool paths from machining strategies, including lacedirection, XY step over, maximum Z increments, feed rates, coolantparameters, and the like, based on the milling machine selected andproperties of the grinding tool, as further described herein.

As illustrated in FIGS. 9A through 9H, a nesting method comprisesarranging the restoration design (900) and the preform body (901) at arotation around line A-A′ which is in the Z-axis direction of a grindingtool when placed in a milling machine. In one embodiment, as illustratedin FIGS. 9A through 9C, a model of the dental restoration design (900)may be nested within the geometry of a preform body (901) so that theincisal edge (902) and the gingival margin (903) of the restorationdesign are adjacent opposite ends, the top end (904) or bottom end(905), of the preform body (901).

In one embodiment, as exemplified in FIGS. 9C through 9H, a nestingoption may be selected based on the location of the stem (907) relativeto the restoration design. The method further comprises designing a stemupper portion of (906) within the body of the preform body (901) thatwill be milled during the restoration milling process. During thenesting process, a position of the restoration (900) around the A-axismay be selected so that the stem upper portion (906) contacts thelingual surface (FIG. 9E, 908) or labial surface (FIG. 9G, 911) of therestoration design. A minimum distance may be established between theouter surface of the stem and the incisal edge or gingival margin.

In an embodiment illustrated in FIGS. 9D and 9E, the restoration designis nested within the geometry of the preform so that the stem upperportion (906) is located lingually, adjacent a mesial side or distalside, such as on, or adjacent, a marginal ridge (917, 917′) between alingual surface and mesial or distal side (909, 910), or near a proximalcontact area with an adjacent tooth. The mesial side as used hereinrefers to a side or surface of the tooth closest to the midline, and thedistal side as used herein refers to a side or surface of the toothfarthest from the midline. A proximal contact area refers to the toothsurface that touches an adjacent tooth in the same arch. A distalproximal contact may contact the mesial side of a more posteriorlypositioned adjacent tooth; the mesial proximal contact may contact thedistal side of an anteriorly oriented adjacent tooth. Contact areas maybe identified by a technician, or automatically identified by therestoration design software.

In embodiments illustrated in FIGS. 9F, 9G and 9H, the restorationdesign is nested within the geometry of the preform so that the upperportion of the stem is located labially, such as between the labialsurface and mesial or distal sides (912, 913), or on a mesial side ordistal side.

In another embodiment, a milling machine is provided with a coolantdelivery system for delivering coolant, such as water, to cool thegrinding tool and/or milling surface during the milling process. Nestingoptions on the lingual or labial surfaces, or adjacent proximal contactareas, optimize for coolant delivery to the cavity (903). In oneembodiment, as illustrated in FIG. 9C and FIG. 9D, a nesting option isselected by aligning the delivery angle of the coolant (914) with thecavity opening (915) of the dental restoration design. In oneembodiment, a nesting option is selected wherein the delivery of thecoolant is directed toward the cavity adjacent the curved lingualsurface (918), as illustrated in FIG. 9C. In an alternative embodiment,a nesting option is selected wherein the delivery of the coolant isdirected toward the side of the cavity adjacent the curved labialsurface. By nesting the restoration design within the preform so thatthe stem contact point is, for example, on a labial or lingual surface,or between a labial or lingual surface and a mesial and/or distal side,the direction of the coolant flow into the cavity of the restoration maybe optimized.

Nesting threshold parameters may be established that provide for aminimum distance between the outer surface of the preform body and theouter surface of the anterior dental restoration design, or between thesurface of the stem and the gingival margin. As illustrated in FIG. 9D,a distance (916) between the stem outer surface and the gingival marginmay be at least 0.5 mm, or at least 1 mm, or between 0.5 mm and 3 mm.Optionally, a distance (917) between the restoration design outersurface and the edge of the preform body may be at least 0.2 mm, or atleast 0.5 mm, or at least 1 mm, as illustrated in FIG. 9C. Further, aminimum distance between the restoration design surface and the stem maybe established, for example, to be about 0.5 mm to 1.5 mm. Therestoration design may also be translated along one or more of A-, B-(along line B-B′) and C-axes of the preform to achieve nestingparameters within the geometry of the preform model. For example,translation along the A-axis adjusts the distance between the incisaledge or gingival margin of a restoration design and top end or bottomend of the preform body. Positional data of the nested restorationdesign may be provided to the CAM system to calculate tool paths toshape the final restoration from the preform.

A computer implemented method for nesting a restoration design isprovided that comprises determining if the restoration fits within thegeometry of a computer model of the preform body; nesting incisal edgeand gingival margin of the anterior restoration design along the A-axis,adjacent either a top or bottom end of a preform body; establishingminimum distance parameters between the restoration design and thepreform body surface, the stem and the restoration surface, and theouter surface of stem and the gingival margin. If the minimum distancebetween the restoration design outer surface and the preform bodysurface cannot be achieved in any nesting position, the program may beexited so that a different option may be pursued by the dentist, such asa modified restoration design or use of an alternative preform body. Ifthe restoration design fits within the preform geometry, at least onenesting option is identified, as follows. The method further comprisespositioning the stem on a labial side, optionally, adjacent a mesial ordistal contact area, or alternatively, on a lingual side, optionally,adjacent a mesial or distal contact area. In one embodiment, the methodfurther comprises determining the angle of the coolant flow andselecting a stem position that optimizes coolant flow onto or into thecavity and maintains minimum distance parameters.

A method is provided for obtaining a machining strategy for a selectednesting position that comprises two or more machining steps to shape arestoration based on the nested design. Each machining step may comprisea tool path for machining a portion of the restoration includingmachining strategy elements such as lace direction, XY step over value,maximum Z increments, feed rates, coolant parameters, and the like. Toolpaths may be established using linear interpolation methods based on XYZmachining positions, and spacing between tool path lines or passes maybe selected to optimize machining conditions.

In an exemplary embodiment of FIG. 10, a tool path (1000) overlaying aportion of the cavity side of a restoration design follows a lacing, orzig-zag, pattern. More than one tool path may be provided for shapingthe restoration. For example, a first tool path may be provided forshaping a portion of the restoration design closest to the gingivalmargin (1001), herein referred to as the restoration cavity side (1003)of the anterior dental restoration. A second tool path may be providedfor shaping a portion of the anterior restoration adjacent the incisaledge (1002), herein referred to as the incisal side (1004) of theanterior restoration. A further tool path may be provided to reduce thediameter of the stem (1005) adjacent the surface of the dentalrestoration to facilitate separation of the stem from the restoration.

Exemplified in FIG. 11A, a lacing pattern (1100) comprises adjacentparallel tool path lines extending across a milling surface as a millingtool (1101) moves in a X-negative (X−) or X-positive (X+) direction,that are separated by a step over distance, for example, in a Y-negative(Y−) or Y-positive (Y+) direction orthogonal the tool path linesrelative to the milling surface. The XY step over distance (1103)distance providing planar spacing between parallel lines of the lacingpattern may be an arbitrary increment, for example, based on tool (1101)dimensions, such as the diameter of the tool tip. Alternatively, anestablished tool path step over distance in a Y-axis direction betweensequential lines may be independently decreased to insert additionallines in the tool path for a portion of the tool path. In oneembodiment, a XY step over is less than 11 μm, or less than or equal to10 μm, such as between 7 μm and 10 μm, or 7 μm and 9 μm.

Controlling material removal by the tool tip or the tool side toadvantageously reduce excess wear on the tool, may be achieved bycontrolling the tool movement in the Z-positive (Z+) direction (movingupwardly relative to a milling surface) or in a Z-negative (Z−)direction (projections through the milling surface). FIGS. 11B and 11Cillustrate cross-sectional representations of the lacing tool path ofFIG. 11A (through the dotted line), and the movement of a tool (1101)across a slope. Step over distances between adjacent lines incrementingin a ‘down-hill’ movement across a slope (FIG. 11B), with projections ofthe tool through the tool path in a Z-negative direction, areillustrated in FIG. 11B. ‘Up-hill’ movement is illustrated in FIG. 11C,as the grinding tool (1101) moves through the tool path towardZ-positive, and the tool is lifted away from the surface to be machined.Movement in the Z-positive direction results in material removal by atool side surface (1104) reducing material removal by a tool tip (1105).Material removal by the tool tip (1105) may result in heating of thetool, wear on the tool tip, and excessive wear on the grinding media,such as diamonds embedded in an alloy coating on the tool shank. Athreshold step over value may be established for a maximum distancebetween two tool path lines in the Z-negative direction (e.g., a maximumZ increment value in a Z-negative direction). Where the distance betweentwo sequential lines in one area of the tool path sequence exceeds amaximum Z-increment, the XY step over distance (1103) between adjacentlines may be decreased to insert additional tool path lines, decreasingthe Z-increment between lines, until the threshold value in theZ-negative direction is met or not exceeded. In one embodiment, amaximum step over between parallel lines in the Z-negative direction isless than 11 μm, or less than 10 μm. In some embodiments, the Z-negativestep over value is between 5 μm and 10 μm, or between 5 μm and 10.5 μm.

Methods are provided that minimize the percentage of Z-negativedirection movement of a grinding tool by the selection of nestingpositions of a restoration design within the preform geometry, and thegeneration of tool paths from the selected nesting position. For eachnesting option, a negative slope value may be calculated for a giventool path. Negative slope values are calculated as the percentage of arestoration design surface area that is determined to have a negativeslope greater than a threshold angle, such as 10°, 15°, 20°, or 30°, orgreater, when viewed from the machining direction. Optionally, anegative slope value may be calculated for a specified surface, such asa portion of the restoration cavity side surface, or a portion of therestoration incisal side surface. A negative slope value may becalculated as the sum of the percentage of surface area having anegative slope greater than a threshold angle for a specified surface.The surface area with negative slope corresponds to the surface area inwhich shaping is performed by down-hill (Z-negative) machining of thetool at an angle greater than or equal to a threshold angle. Forexample, in one embodiment using an stl. file format, a surface of arestoration design may be analyzed to determine what percentage of atriangulated surface geometry is sloped greater than a threshold valuerelative to normal, when viewed from a machining direction. A nestingposition having a low negative slope value, corresponding to thepercentage of surface area with negative slope less than or equal to aspecified threshold value, may be selected as the nesting position fromwhich to calculate a tool path. Nesting software may be separate fromthe dental restoration design software or may comprise a module of thedesign software that may be automatically implemented upon completion ofthe restoration design. Nesting information, comprising positional dataof the restoration relative to the preform body, and the stem relativeto the dental restoration, may be used for computing tools pathsequences.

Machining strategies based on selected nesting options may comprise toolpaths that are split based on dimensional limits of the milling machineand tool. As illustrated in FIG. 10, a flat separator (1006) betweenrestoration incisal side (1004) and cavity (1002) side coincides withthe dimensional limits of a 3-axis milling machine and the milling toolwhere the restoration surface angles (1009) away from the parting linealong the Z-axis, when viewed from the restoration cavity side. In oneembodiment, cavity side and incisal side machining steps are separatedat a point (1007) of the Z-axis (1008) coincident with a parting linebetween incisal and cavity sides that is farthest from the gingivalmargin.

In one embodiment, FIGS. 12A through 12G further illustrate a machiningstrategy for a preform (1201) that is split at a flat separator (1205)between the cavity side (1201) and incisal side (1213). The cavity side(1201) of the restoration design may be split into two or more machiningsteps (1200, 1210) having separate tool paths. In one embodimentillustrated in FIG.12A, a first machining step (1200) having a firsttool path is provided to shape a first portion of a cavity side (1206)of the restoration that is opposite the stem (1207), and a secondmachining step (1210) having second tool path shapes a second portion ofa cavity side (1212) that is adjacent the stem. Tool path tool entrypoints (e.g., 1203, 1211) and a tool stop point (e.g., 1204) are shownin FIGS. 12A and 12C.

The tool path sequence for machining first and second portions of thecavity side may be split at an arbitrary point relative to the Y-axis,or the tool path sequence may be split based on positional informationof the nested restoration, selected to optimize machining parametersdescribed herein. In one embodiment, as exemplified in FIGS. 12D and12E, first and second cavity side tool paths are separated at a point(e.g., 1214) relative to the Y-axis in which the surface of the firstcavity side opposite the stem slopes negatively toward the stem (1207)greater than a maximum angle. In one embodiment illustrated in FIGS. 12Fand 12G, the first and second portions of the cavity side compriselacing tool paths with line lengths extending in the Z-axis directionover the vestibular surface, or perpendicular to the flat separator(1205), minimizing material removal by the tool tip and maximizingmaterial removal by the tool side surface. In a further embodiment, asillustrated in FIG. 12F a single lacing tool path for the first portionof the cavity side (1206, opposite the stem) is applied to most of thecavity side vestibular surface and the fitting surface of the cavity,and a second lacing tool path, illustrated in FIG. 12G, may be providedfor the second portion of the cavity side (1212, adjacent the stem)which consists of the remainder of the cavity side vestibular surfaceand optionally, a portion of the stem.

Further, a method is provided for separating a tool path sequence forthe incisal side of an anterior dental restoration that is nested, forexample, according to FIGS. 9A and 9B. In some embodiments, tool pathsfor milling the incisal side of the restoration (1300) are separated atthe incisal edge (1303), for example, parallel (1302) or orthogonal(1304) to the incisal edge, to minimize material removal in theZ-negative direction. FIGS. 13A through 13D illustrate an anteriorrestoration design (1300) nested within a preform body (1301), andmethods for separating first and second tool paths of an incisal sidecomprises detecting a topmost point (1305) on the incisal edge of thecrown. Angles (e.g., 1306, 1307) formed from lines (e.g., 1308, 1308′)intersecting a vertex (1313) at a distance (e.g., 1 mm) above thetopmost point (1305) and extending to points (e.g., 1309, 1310) onopposite outer surfaces of the design, may be identified to determinethe smallest angle. The smallest angle may be selected and a linebetween the opposite points of the intersecting lines may be usedseparate tool paths. In one embodiment, opposite points at mesial anddistal surfaces (1309, 1310) of intersecting lines (1308, 1308′) definea line of separation (1302) parallel to the incisal edge (1303) thatseparates tool paths between lingual and labial surfaces, as illustratedin FIGS. 13A and 13D. In an alternative embodiment, opposite points atlingual (1311) and labial (1312) surfaces of intersecting lines (1214,1214′) define a line of separation (1204) that is orthogonal the incisaledge (1203) separating the tool paths between mesial and distal sides ofthe restoration, as illustrated in FIG. 13C.

As exemplified in FIGS. 14A and 14B, first and second lacing tool pathsseparated at an incisal edge (1403) are illustrated for shaping theincisal side of an anterior restoration design. A first lacing tool pathis provided for the labial (1401) surface and a second lacing tool pathis provided for the lingual (1402) surface. In one embodiment, machininginstructions for shaping the incisal side of a dental restorationcomprise a first tool path for shaping the labial surface (1301) and asecond tool path for shaping the lingual surface, wherein both toolpaths have sequential tool path line lengths parallel the incisal edge(1403), that extend between mesial and distal sides of the tooth to theparting line.

Tool paths may follow the parting line (1404) between the incisal edgeand the gingival margin, and extend to tool offset positions (e.g.1405). The parting line (1404) follows the contour, or largest dimensionof the outer surface, of the restoration when viewed from the incisaledge. The first and second tool paths may overlap the incisal surface orthe parting line for a distance of, for example, 0.3 mm to 2 mm toprovide a smooth transition. Tool offset positions may be calculatedbased on the dimensions of the preform body and stem.

A method is also provided for reducing the stem geometry to facilitateseparating the stem from the milled restoration. As exemplified andillustrated in FIGS. 15A, a rotary tool path around a first stem end(1501) adjacent the dental restoration (1500) reduces the stem diameterto enable the restoration to be snapped off the stem by hand or by ahandheld tool. In one embodiment, machining steps comprising a rotarytool path (1502) around the stem form a plurality of horizontal slicesthat are perpendicular to the length axis of the stem. The distance ofthe tool path boundary from the stem may increase where the stemattaches to the restoration on an angled surface of the restoration. Forexample, as illustrated in FIG. 15B, a boundary (1503) for a rotary toolpath that extends to reach a point (1504) of the crown high along theY-axis, may unnecessarily extend over a portion of the restorationprofile where the crown is lower on the Y-axis (1505).

In an alternative embodiment as illustrated in FIGS. 16A and 16B, arotary tool path (1601) is provided wherein rotary movement of themilling tool around the stem (1602) is synchronized with movement of thetool in the Y-axis direction. To reduce the size of the tool pathboundary, a rotary tool path is provided where as the tool rotatesaround the stem, Y-axis direction tool path movement (1604)simultaneously follows the restoration surface (1603) that is adjacentthe stem. In one embodiment, as illustrated in FIGS. 16C and 16D, arotary tool path further comprises a containment border (1605), wherethe boundary of the rotary tool path does not extend past thecontainment border. The diameter of the containment border (1605) may becalculated as the sum of the diameter of the stem at the surface of therestoration and the radius of the milling tool (for example, 0.5 mm to 1mm, such as 0.8 mm), and optionally, a tolerance distance may be addedto the diameter. The distance of the movement of the tool in the Y-axisdirection during a rotation may be increased or decreased so that theboundary of the tool path complies with the containment border.

In one embodiment, a method for generating machining instructions for ananterior restoration comprises: 1) nesting an anterior restorationdesign within a computer model of a preform body wherein the preformbody comprises a cylindrical shape having a stem projecting from thecurved outer surface, wherein nesting comprises: a) positioning theincisal side of the restoration design adjacent a first circular end ofthe preform and the cavity side adjacent the second circular end; and b)positioning a first stem end adjacent a labial or lingual side or arestoration design, optionally adjacent or on a mesial or distal side;and 2) generating machining instructions comprising a) a first tool pathfor shaping the cavity side of an anterior restoration, b) a second toolpath for shaping the incisal side of a restoration, and c) a third toolpath for reducing the diameter of the stem. In one embodiment, thenesting step of positioning the first stem end adjacent the restorationdesign comprises aligning the fitting surface of the restoration designin the direction of the flow of coolant from the milling machine.

In a further embodiment, the method comprises generating machininginstructions having at least five tool paths comprising: a) a first toolpath for machining a first portion of the cavity side of the restorationthat is opposite the stem, b) a second tool path for machining a secondportion of the cavity side of the restoration that is adjacent the stem,c) a third tool path for machining a first portion of the incisal sideof the restoration corresponding to the labial surface, d) a fourth toolpath for machining a second portion of the incisal side of therestoration corresponding to the lingual surface, and e) a fifth toolpath for reducing the diameter of the stem. In one embodiment, toolpaths for machining a portion of the cavity side and the incisal sideare lacing tool paths.

In a further embodiment, the method comprises splitting first and secondincisal tool paths along a line that is substantially parallel theincisal edge, wherein the first incisal side comprises the labialsurface adjacent the incisal edge and a second incisal side comprisesthe lingual surface adjacent the incisal edge. Optionally, the lacingtool paths comprise sequential lines that are parallel the incisal edgefor the labial surface and the lingual surface. In a further embodiment,a tool path for milling a portion of the cavity side opposite the stemencompasses a portion of the vestibular surface and the fitting surface.In another embodiment, the fifth tool path for reducing the stemcomprises a rotary tool path having simultaneous movement in a Y-axisdirection, wherein the tool path is contained within a border in therotary direction.

The order of machining steps and tool paths may vary, and terms such asfirst and second, for example, as used in first tool path, second toolpath, third tool path, first machine step, second machine step, and soforth, are used for descriptive convenience, and should not be connotedas indicative of a specific order of steps unless otherwise noted.

Material feed rates may be individually controlled for each machinestep. Machining parameters may comprise different material feed ratesfor incisal side and cavity side tool paths. Machining parameters may beimplemented on a 3+1 axis CNC machine to shape a finished anteriordental restoration from a ceramic block comprised of material having aVickers hardness value greater than or equal to about HV4 GPa with asingle grinding tool comprising an alloy coating embedded with diamondsin a chair-side application. In another embodiment, the custom dentalrestoration may be machined in a 3+2, or 4, or 5 axis machine.Corresponding 3+2 or 4 or 5 axis machining cycles may be used to specifya tool axis angle relative to the tool contact normal of the machinedsurface either directly from the CAD data of the restoration, orindirectly using a separate tool axis drive surface interpolated fromthe original CAD data. In a further embodiment, more than one grindingtool may be used for grinding the preform. Multiple grinding tools maybe used sequentially, for example, for roughing and finishing, ormultiple grinding tools may be used simultaneously, on opposite surfacesof the preform. A suitable chair-side milling machine includes, but isnot limited to the TS150™ chair-side milling system (IOS Technologies,San Diego, Calif.), and an exemplary grinding tool (300) suitable foruse herein is illustrated in FIG. 17.

The methods described herein provide enhanced machining in chairsideapplications for preform materials having a Vickers hardness valuegreater than or equal to about HV 4 GPa (Vickers macro-hardness), or avalue in the range of HV 4 GPa to HV 20 GPa (i.e., HV 4 GPa to HV 20GPa), when measured according to the method provided herein.Alternatively, preform materials have a Vickers Hardness value betweenHV 5 GPa and HV 15 GPa, or between HV 11 GPa and HV 14 GPa. Preform bodymaterials comprising hardness values within this range may includemetals, such as cobalt chrome, glass and glass ceramics, such as lithiumsilicate and lithium disilicate, and ceramic materials, includingsintered ceramics comprising alumina and zirconia.

Dental restoration materials, including but not limited to commerciallyavailable dental glass, glass ceramic or ceramic, or combinationsthereof, may be used for making the preforms that are machinable by themethods described herein. Ceramic materials may comprise zirconia,alumina, yttria, hafnium oxide, tantalum oxide, titanium oxide, niobiumoxide and mixtures thereof. Zirconia ceramic materials include materialscomprised predominantly of zirconia, including those materials in whichzirconia is present in an amount of about 85% to about 100% weightpercent of the ceramic material. Zirconia ceramics may comprisezirconia, stabilized zirconia, such as tetragonal, stabilized zirconia,and mixtures thereof. Yttria-stabilized zirconia may comprise about 3mol % to about 6 mol % yttria, or about 2 mol % to about 7 mol % yttria.Examples of stabilized zirconia suitable for use herein include, but arenot limited to, yttria-stabilized zirconia commercially available from(for example, through Tosoh USA, as TZ-3Y grades). Methods form makingdental ceramics also suitable for use herein may be found in commonlyowned U.S. Pat. No. 8,298,329, which is hereby incorporated herein inits entirety.

The preform body may be made from unsintered materials shaped into anintermediate form having substantially the same geometry as the sinteredpreform, but with enlarged dimensions to accommodate shrinkage uponsintering, where necessary. Suitable unsintered ceramic materials may bemade into blocks by processes including molding and pressing, includingbiaxial or iso-static pressing, and may optionally comprise binders andprocessing aids. Ceramic blocks may be shaded so that the sinteredpreforms have the color of natural or artificial dentition, requiring nofurther coloring after formation of the dental restoration. Coloringagents may be incorporated during block formation to more closely matchthe appearance of natural or commercially available artificial dentitionthan uncolored or unshaded ceramic materials. Optionally, ceramic powdermay be processed into blocks by slip casting processes, includingprocesses described in commonly owned U.S. Patent Publication No.2009/0115084, U.S. Pat. No. 9,365,459, and U.S. Pat. No. 9,434,651, eachof which is hereby incorporated herein by reference in the entirety.Pre-sintered ceramic blocks suitable for use in making intermediateshaped forms include commercially available ceramic milling blocksincluding those sold under the trade name BruxZir® (for example,BruxZir® Shaded 16 Milling Blanks, Glidewell Direct, Irvine, Calif.). Insome embodiments, the theoretical maximum density of fully sinteredzirconia ceramics is between about 5.9 g/cm³ to about 6.1 g/cm³, or forexample, or about 6.08 g/cm³.

A unitary preform may be shaped from a from a single continuousgreen-state block or pre-sintered ceramic block, requiring no separateattachment step for attaching the stem and/or attaching member to thepreform body. Alternatively, the preform may be made by known moldingprocesses, including injection molding. The intermediate shaped form maybe sintered to a density greater than about 95% of the theoreticalmaximum density by known sintering protocols. Sintered zirconia ceramicpreforms may have densities greater than about 95%, or greater thanabout 98% or greater than about 99%, or greater than about 99.5%, of themaximum theoretical density of the ceramic body.

The preform body comprises materials that are shapeable into dentalrestorations in chair-side applications by the methods described herein,that have acceptable strength properties for use in anterior, posterioror both anterior and posterior dental restoration applications, withoutadditional post-shaping processing steps to alter the material strengthproperties after shaping, such as by sintering. Sintered preforms maycomprise zirconia ceramic materials that have high flexural strength,including strength values greater than about 400 MPa, or greater thanabout 500 MPa, or greater than about 600 MPa, or greater than about 800MPA, when tested by a flexural strength test method for zirconiamaterials as outlined in ISO 6872:2008, as measured and calculatedaccording to the 3 point flexural strength test described forDentistry—Ceramic Materials, or as provided herein.

Flexure strength testing may be performed on sintered test materialsusing the Instron—Flexural Strength test method for zirconia materialsas outlined in ISO 6872:2008. Test bars may be prepared by cuttingbisque materials taking into consideration the targeted dimensions ofthe sintered test bars and the enlargement factor (E.F.) of thematerial, as follows: starting thickness=3 mm×E.F.; starting width=4mm×E.F.; starting length=55 mm×E.F. The cut, bisque bars were sinteredsubstantially according to the sintering profile provided bymanufacturer of the bisque material. Flexural strength data was measuredand calculated according to the 3 point flexural strength test describedin ISO (International Standard) 6872 Dentistry—Ceramic Materials.

Preform materials may be tested for hardness using a Vickers Hardness(macro-hardness test). Hardness numbers (HV) may be calculated asdescribed in ISO-6507, or by determining the ratio of F/A where F is theforce applied in kg/m² and A is the surface area of the resultingindentation (mm²). HV numbers may be converted to SI units and reportedin units, HV GPa, as follows: H(GPa)=0.009817 HV.

Dental restorations may be made by grinding sintered ceramic preformsusing grinding tools instead of traditional milling tools because of thematerial hardness which renders typical milling tools unsuitable incertain embodiments. Grinding tools having a diamond coating, includingnickel plated tools embedded with diamonds, are suitable for use herein.A grinding tool (1700) having a shank (1701) and tool tip (1702), forexample as illustrated in FIG. 17, comprises an embedded diamond coatingon the shank (1701) and tip. Diamonds suitable for use herein includeblocky or friable diamonds having an average size in the range of about90 micron to about 250 micron, or an average size in the range of about107 micron to about 250 micron, or an average size in the range of about120 micron to about 250 micron, or for example, an average size in therange of about 120 micron to about 180 micron. Suitable diamond coatingsinclude those in which at least 50% of the diamonds are embedded by ametal alloy layer for more than half the height of the diamond, forexample, as determined by SEM analysis. Grinding tools having a coatingin which diamonds are embedded in a metal alloy to a depth of about 50%to 95% of the average diamond size, or about 60% to about 95% of thediamond size, or to about 80% to about 95% of the diamond size areuseful for shaping preforms made from materials such as fully sinteredzirconia preforms, or preforms comprising materials having the hardnessvalues described herein. In some embodiments, grinding tools have adiamond coated shank with a metal alloy layer having thickness that isgreater than about 50% of the diamond grit size (e.g., in microns), orgreater than about 60%, or greater than about 70%, or greater than about80%, or greater than 90%, or between about 60% and 90%, or between about80% and 100%, of the diamond grit size (e.g., in microns). In oneembodiment, a grinding tool has a diamond coated shank comprising adiamond size in the range of 126 grit to 181 grit, and a nickel alloylayer having a thickness that is greater than or equal to about 70% ofthe diamond grit size (in microns).

Examples 1 and 2

Sintered zirconia preforms and polymer mandrels were prepared havinginterference fit connections, and maximum load of the connection wasmeasured and averaged for multiple samples.

Forty-six sintered zirconia ceramic preforms were prepared having aconfiguration substantially the same as FIGS. 7A and 7B, by milling andsintering yttria-stabilized zirconia bisque stage blocks. Forty-sixnylon mandrels were made having the configuration substantially the sameas FIG. 6. The sintered zirconia preform stem ends comprised twoopposing ridges projecting above the outer diameter of the smooth curvedportions of the stem end, with channels (707) on either side of theridges (706) and a concentric groove between ridges. The inner surfaceof the top of the mandrels accessible through a hole was smooth and theinner diameter of each was smaller than the diameter of the stemmeasured through the ridges. A two-part adhesive (UHU PLUS 300, USUGmbH, Búhl, Germany) was applied to the outer surface of the stem endfilling channels for half of the samples. The stems were inserted intothe mandrels by applying pressure to deform the inner surface of thesofter mandrel material with the ridges to achieve an interference fit,and glued samples were allowed to dry at room temperature.

The strength of the connection between mandrel and preform was tested onan Instron machine measuring pulling (axial) force reported as Max Load(N). Glued samples had an average maximum load of 521 N (23 samples) andnon-glued samples had an average maximum load of 245 N (23 samples).

We claim:
 1. A method for making an anterior dental restoration from afully sintered zirconia preform, comprising obtaining an anterior dentalrestoration virtual design and a virtual model of a preform body,wherein the preform body comprises a cylindrical shape having a stemprojecting from a curved outer surface; nesting the anterior dentalrestoration virtual design within the virtual model of the preform;generating machining instructions for shaping an anterior dentalrestoration from a fully sintered zirconia preform, said machininginstructions comprising: a first tool path for shaping a cavity side ofthe anterior dental restoration, a second tool path for shaping anincisal side of the anterior dental restoration, and a third tool pathfor reducing a diameter of a stem of the fully sintered zirconiapreform; and machining the fully sintered zirconia preform using themachining instructions.